Bakken, Inger¹, Ruth Elin A. Midtbø¹, Johannes M. Rykkje¹
(1) Norsk Hydro, 5020 Bergen, Norway

ABSTRACT: The Effect of Porosity Preserving Clay Coating on Permeability: Examples from Deeply Buried Sandstone Reservoirs Offshore Norway

Coatings on quartz grains such as clay rims and chlorite, have been shown to preserve porosity. Unusually high porosity values have been measured in deeply buried sandstone reservoirs where quartz cementation has been inhibited by grain coating.
Good reservoir quality depends on sufficient permeability as well as high porosity, therefore porosity preservation alone is not enough to generate a high-quality sandstone reservoir at deep burial conditions. Even though many authors have given evidence that good reservoir quality at deep burial is associated with well developed chlorite coats, the nature of the association is complex. Although uncoated samples never display high porosity and permeability at these depths, there is a wide range in permeability within similar porosity values among coated samples.
Examples will be given of different types of clay coatings that preserve porosity but have different impact on permeability. The samples are core samples taken from deep reservoir wells off South- and Mid-Norway. All the samples are from facies below 3000m where grain coating chlorite could be expected, but where the illite-chlorite assemblage also is a characteristic pair in the diagenetic regime. Though the samples have measured porosity values >20%, the permeability values range from 0.1mD to 10000mD. The porosity preserving clay coatings have been studied regarding morphology, thickness, crystal size and chemistry, and related to permeability.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.